1. Field of the Invention
The present invention relates to a driving apparatus using an electromechanical transducer such as a piezoelectric element, an electrostrictive element, or a magnetostrictive element. More particularly, the present invention relates to a driving apparatus in which a driving friction member is bonded to an electromechanical transducer by adhesive.
2. Description of Related Art
Heretofore, there has been used a driving apparatus for moving a movable unit by utilizing an electromechanical transducer such as a piezoelectric element, an electrostrictive element, or a magnetostrictive element. In such driving apparatus, for example, the electromechanical transducer is fixed at one end to a fixed member and at the other end to a driving friction member, respectively, and a movable unit is slidably mounted on the driving friction member with friction. When the electromechanical transducer is slowly deformed, the movable unit is moved along with the driving friction member by friction. When the electromechanical transducer is quickly deformed, on the other hand, the movable unit remains at the same place by inertia. This driving apparatus is used to produce a requested movement of the movable unit by appropriately combining those slow and quick movements.
Herein, the electromechanical transducer and the driving friction member are commonly coupled by adhesive. These have conventionally been designed to have bonded areas as large as possible in order to prevent the adhesive from easily peeling due to deterioration from vibrations for driving, long-term use, or other causes. For example, Japanese unexamined patent publication No. H8(1996)-286093 (FIG. 3) discloses a driving apparatus in which a piezoelectric element and a driving friction member are integrally adhered and fixed so that a coupling part of them is entirely covered with a reinforcing member.
In the driving apparatus disclosed in the '093 publication, however, the reinforcing member is overlaid on the driving friction member, so that the actually available length of the driving friction member would be reduced by just that much. If the reinforcing member is simply eliminated, on the contrary, the apparatus is provided as a driving apparatus 100 shown in FIG. 5. In this driving apparatus 100, a driving friction member 102 and a fixed member 103 are bonded to both ends of a piezoelectric element 101 respectively by adhesive 120. The driving friction member 102 is held to be movable in its axial direction by bearings 104 and 105.
In such conventional driving apparatus 100, the adhesive 120 applied between the piezoelectric element 101 and the driving friction member 102 creeps, due to its fluidity, leftward in the figure along the periphery of the driving friction member 102. The creeping amount varies with an application amount and an application position of the adhesive 102. Accordingly, it would be necessary to determine the placement of the bearing 105 in consideration of the creeping amount. Specifically, the distance S from a bonded surface 121 to a right end of the bearing 105 is determined by adding a maximum creeping amount of the adhesive 102 to a maximum expanding amount of the piezoelectric element 101. In this figure, the adhesive 102 that crept is shown very exaggeratedly. However, even the adhesive that crept very thinly would also interfere with the bearing 105.
Consequently, an effective length Q of the driving friction member 102 is a length obtained by subtracting the distance S and each width of the bearings 104 and 105 from a whole length P of the driving friction member 102. This effective length Q is a range where a movable unit can frictionally be mounted on the driving friction member 102, that is, a movable range of the movable unit. In recent years, there is an increasingly demand to further reduce the entire size of the driving apparatus 100, particularly, the axial length. However, the reduction in the effective length Q is undesirable in view of driving performances.